Orographic impacts of Réunion Island and Madagascar on heavy rainfall during Tropical Cyclone Batsirai (2022)

Abstract. During the passage of tropical cyclone (TC) Batsirai (2022), the mountainous regions of Réunion Island and the eastern part of Madagascar experienced devastating floods. TC Batsirai passed north of Réunion Island (minimum distance of ~210 km) before making landfall along the eastern coast of Madagascar. Using the Meso-NH model at 2 km resolution, we conducted a control simulation with realistic topography (CTL) and three flattened-terrain experiments (FLT_all, FLT_mdg, and FLT_reu) to assess orographic effects. Results show that the terrain of Réunion and Madagascar enhanced localized rainfall by 94 % (2,800 mm) and 82 % (830 mm), respectively. Over Réunion Island, orographically uplifted low-level moisture sustained 60 h of heavy rainfall (10 mm h^‒1), with a peak of 96 mm h^‒1, although the cyclone’s intensity and track were minimally affected. In Madagascar, terrain not only intensified precipitation (19 h of heavy rainfall with a peak intensity of 92.8 mm h^‒1) but also altered the TC trajectory, delaying landfall by about 12 h and shifting it approximately 30 km south. These findings highlight the critical role of complex orography in shaping localized rainfall and TC behaviour in the southwest Indian Ocean.